I was never going to buy an Xbox One despite some exciting exclusives releasing on it, so it's great to hear that games like Gears 4, Forza and Scalebound are coming to PC. But those are some high quality exclusives. How is this going to convince people to buy an Xbox One? Because Xbox One is now basically a mediocre PC with only a few exclusives and possibly none soon if any game gets the Play Anywhere treatment in the future. I've the idea I'm missing something, because this would be a terrible marketing decision. The Xbox One is now worth less to me. 

If you were never going to buy one it was already useless to you come on dude. There's always been PC gamers. There will always be PC gamers just like there will always be Console gamers.

Xbox as a service. If you have both play on both for no extra cost as you like. If you play on PC you do, if you play on consoles, you do. Xbox wont be a matter of platform soon.

Well, now there's more people to play with.
Also to buy a PC that can play these games will cost more than a regular xbox one. The xb1 is still pretty cheap compared to a PC.
Overall games coming to PC means that MS makes more money and more money means more games and more games mean xbox one and w10 exclusives.

Its not about the hardware, its about the OS and middleware.

Going forward its very likely that there will be no distinction between PC and XB1 as its all based on a WindowsOS with the same uniform middleware such as Unity.

The most important question is how will MS benefit from it. Seems like a way to push W10 while at the same time expanding their audience.

I think it will benefit in the long term probably closer to Scorpio launches by then the Windows 10 store will very likely be full up on 3rd party games like steam/gog and eventually this will mean games on that are on PC will be playable on the console through play anywhere,

This could also potentially be a way to get the Japanese games back onto a Xbox console that have been released for PS4/PC as this trend is getting more and more popular among Japanese devs.

This plus pushing the more and more unified store. It's all about selling software (and today services). That goes for consoles as well.

I don't do PC gaming, so I have to buy a XB1 to play the games on it.

I don't think Play Anywhere alone does a lot for the Xbox. At most it makes it so that PC gamers pretty much have a defacto device for their tv, since all of their games will work across all Windows devices now.

I think the bigger thing for Xbox is the unification of the Windows and Xbox stores. With Windows Universal Apps, the XBO is basically the ultimate PC for the casual gamer, or the light average joe computer user.

Microsoft is in a very good position right now. They just need to get the UWAs in the store to start closing the gap with the app store and google play, and they will be a dangerous player in every market.